The present invention relates to a linearizer for a power amplifier and in particular but not limited to a linearizer for a microwave power amplifier,
In modern communication systems, linearized power amplifiers have found wide use. This is mainly due to the availability of highly efficient power amplifiers (PA), spectral efficient digital multi-state modulation techniques, and the requirement fox multi-carrier operation. Digital multi-state signals and multi-carrier signals are very sensitive to non-linearities of the PA, which cause unwanted interference to be produced. The unwanted interference is mainly produced by Amplitude Modulation to Amplitude Modulation (AM-AM) conversion and Amplitude Modulation to Phase Modulation (AM-PM) conversion in the power amplifier. A traditional way to reduce the unwanted interference is to back-off the PA from its saturated output power. Back-offs of 10 to 20 dB are common which causes the power efficiency of whole system to be greatly reduced. The unwanted interference generated by the amplifier non-linearities can be especially serious if multiple carriers or multi-state digitally modulated signals are applied to the PA. This is because the peak power of multiple carriers and the multi-state digitally modulated signals are much higher than their average powers. Commonly, the peak power can range from 3 to 20 dB or higher than the average power depending on the numbers of carriers and modulation technique. These higher peak powers cause the signals to enter the saturated regions of the PA sooner and increase their sensitivity to the non-linearities of the PA. In order to increase power efficiency of the system, linearization techniques must be applied in the PA. Solutions so do this include feedback techniques, Cartesian feedback, feedforward, predistortion, LINC linearized transmitter, Kahn envelope elimination and restoration (EER) techniques and many other techniques.
Among these techniques, predistortion is the most popular, especially the RF analogue predistorter that is suitable for application to the high frequency and millimeter wave bands. Many different types of predistorters have been proposed. One implementation employs two variable gain amplifiers as described in M. Kumar, J. C. Whartenby and H. J. Wolkstein, xe2x80x9cPredistortion Linearizer Using GaAs Dual-Gate MESFET for TWTA and SSPA Used in Satellite Transpondersxe2x80x9d, IEEE Trans. On MTT, Vol. MTT-33, pp. 1479-1488, No. 12, 1985, and U.S. Pat. No. 5,576,660. However, there are still some problems concerning bandwidth and cost because hybrids limit the bandwidth and many components are used to adjust the predistorter. All of these can increase the cost. Another problem that limits the bandwidth is the memory effect caused by the predistorter. Even though two ideal flat variable gain amplifiers are applied, the predistorter can still have a slope over the bandwidth. The slope will significantly affect linearization in wideband applications.
In the implementation described by M. Kumar, J. C. Whartenby and H. J. Wolkstein, xe2x80x9cPredistortion Linearizer Using GaAs Dual-Gate MESFET for TWTA and SSPA Used in Satellite Transpondersxe2x80x9d, IEEE Trans. On MTT, Vol. MTT-33, pp. 1479-1488, No. 12, 1985, two dual gate MESFET variable gain amplifiers are adopted. It was realized in a hybrid Microwave Integrated Circuit (MIC) technique, that many tunings are needed to ensure that the predistorter produces proper distorted signals, especially for the phase difference between the linear path and non-linear path. It is a difficult thing to choose two amplifiers that can satisfy the phase difference requirement. Another problem concerning the implementation is the bandwidth. Normally, a 90xc2x0 hybrid has limited bandwidth of only 10%. If 20% bandwidth or more is needed, a new design is required and this will increase design costs.
In the implementation described in U.S. Pat. No. 5,576,660, two identical voltage controlled phase shifters, two identical voltage controlled attenuators and two identical amplifiers are used. Two voltage controlled phase shifters make it relatively easy to control the phase difference between the linear path and the nonlinear path. But they are costly to manufacture, especially for the application to mm-wave band. The predistorter is the same as the first realization in that a 90xc2x0 hybrid is adopted This hybrid limits the bandwidth if wideband is needed.
According to one aspect of the present invention, there is provided a predistortion linearizer comprising a signal separator for receiving a signal and for dividing the signal into a first signal and a second signal, a first path for caring the first signal, a second path for carrying the second signal, a signal combiner for combining the signals from the first and second paths, the second path between the separator and combiner including a non-linear device for receiving the second signal and generating distortion therefrom, and a vector modulator in one of the first and second paths for controlling at least one of the amplitude and phase of the signal carried by the respective path, the vector modulator comprising a second signal separator for dividing the signal carried by the respective path into a first modulator signal and a second modulator signal, a first modulator path for carrying the first modulator signal, a second modulator path for carrying the second modulator signal, and a second signal combiner for combining the first and second modulator signals received from the first and second modulator paths, the vector modulator including means for producing a phase difference between the first modulator signal and the second modulator signal, and wherein the first modulator path includes amplitude controlling means for controlling the amplitude of the first modulator signal, and the second modulator path includes amplitude controlling means for controlling the amplitude of the second modulator signal.
Advantageously, in this arrangement of a predistortion linearizer, the means for controlling the amplitude and chase of the distorted signal is provided by vector modulator means which allows the predistortion linearizer circuit to be considerably simplified and the number of components and required control signals to be reduced over known predistortion linearizers.
In one embodiment, the vector modulator may include means for dividing the distorted signal into first and second modulator signals, means for providing a phase difference between the two signals, and means for varying the amplitude of each signal independently of the other, and a signal combiner to combine the first and second modulator signals. Advantageously, this arrangement enables the phase and amplitude of the distorted signal to be varied independently of one another using just two control signals, each of which is used to control the amplitude of the modulator signals.
In one embodiment, the signal separator may comprise a wideband separator, for example having a bandwidth of at least 20% of the center frequency of the input signal. The signal combiner may also be a wideband signal combiner having a bandwidth of for example at least 20% of the center frequency. Advantageously, this enables the predistortion linearizer to operate over a wide frequency range, enabling a single design of device to be used in a wide range of different applications, thereby enabling the design and manufacturing costs to be reduced.
A delay line may be incorporated in one of the signal paths in order to provide a phase difference between the first and second signals, in addition to that that can be provided by the vector modulator.
According to another aspect of the present invention, there is provided a vector modulator comprising a signal separator for dividing a signal into a first modulator signal and a second modulator signal, a first path for carrying the first modulator signal, a second path for carrying the second modulator signal and a signal combiner for combining the first and second modulator signals received from the first and second paths, wherein the first path includes means for changing the phase of the first modulator signal and means for controlling the amplitude of the first modulator signal, and the second path includes means for controlling the amplitude of the second modulator signal.
This arrangement enables a vector modulator to be realized having just two variable attenuators to independently control the phase and amplitude of the signals, thereby providing a vector modulator which has few components and can be designed and manufactured at low cost.
In one embodiment, the means for changing the phase between the first and second modulator signals comprises a delay line, which may he implemented simply by making one path longer than the other.
The signal separator and combiner may have any desired bandwidth to enable the vector modulator to operate at the desired frequency and over the desired frequency band.
According to another aspect of the present invention there is provided a predistortion linearizer for a power amplifier comprising: a signal separator for receiving a signal and for dividing the signal into a first signal and a second signal, a first path for carrying the first signal, a second path for carrying the second signal, the second path comprising a non-linear device for receiving the second signal and generating distortion therefrom, amplitude control means in at least one of the first and second paths for controlling the amplitude of the signal carried by the respective path, a variable length delay line in one of the first and second paths for enabling a phase difference between the first and second signals to be varied, and a signal combiner for combining the signals from the first and second paths into a signal for feeding into a power amplifier.
Advantageously, in this arrangement, the phase difference of the signals in the first and second paths at the input of the signal combiner is produced by arranging one path longer than the other. In other words, it is the difference in path length between the first and second paths which is used to produce the phase difference of the input of the combiner. Advantageously, this arrangement obviates the need for relatively complex and expensive phase shifters and phase shift compensators employed in the prior art arrangements, and offers an inexpensive means of producing the required phase shift.
In one embodiment, the additional length in one of the first and second oaths is provided by a delay line. The delay line may comprise a variable length delay line to enable the phase to be varied.
In one embodiment, the additional length is in the first path.
In one embodiment, the means for attenuating may be arranged for attenuating the input power of the first signal to the amplifier.
In one embodiment, the means for attenuating comprises a variable attenuator, for example, a voltage controlled attenuator.
In one embodiment, the means for attenuating the power of the distorted signal from the non-linear device comprises a variable attenuator.
In one embodiment, the signal separator comprises a wideband signal separator. Advantageously, this assists in enabling the same linearizer to be used over a wide range of different frequencies.
In one embodiment, the signal separator has a bandwidth of at least 20% of the center frequency of the input signal.
In one embodiment, the signal combiner comprises a wideband coupler.
In one embodiment, the wideband coupler has a bandwidth of at least 20% of the center frequency of the input signal.